Microstructure-based interior cracking behavior of α + β titanium alloy under two stress ratios and intermediate temperature in the very high-cycle fatigue regime

Axial loading fatigue tests were conducted for α + β titanium alloy with two stress ratios to elucidate the microstructure-based interior cracking behavior at 150 °C temperature. The interior failure is attributed to the cleavage of the large primary α-grains (α_p) and facet–facet cluster zone–fisheye formation, which is the primary failure mode in the very high-cycle fatigue regime. Furthermore, microcracks are generated through soft-oriented large α_p-grains along the {0001} slip plane in the direction of maximum shear stress, forming the crystallographic facets. Moreover, due to the variation in the grain orientation, significant plastic deformation is produced within the facet-cluster zone. The low-angle grain boundaries offer limited resistance in the propagation of the formed crack, which can pass through neighboring grains easily, exhibiting characteristic transcrystalline failure. Based on the analysis above, the interior fracture mechanism was summarized as related to the microstructure characteristics. In addition, dislocation configurations, slip bands, and stacking faults indicate that the deformation behavior with faceting-induced fracture occurs by joint action of dislocation bypassing, stacking fault shearing, and increasing thermal activation energy at intermediate temperature. The microstructure-based interior failure mechanism is presented based on electron backscatter diffraction, focused-ion beam, and transmission electron microscopy.